The ZROCK2 works into impedances of 24K or higher.  I determined this by first measuring the unit into a 200K load at the sweet spot where I typically have mine set.  Since it ends up set in this position so often, I marked it with a white dot.  This came in handy when I decided to measure the unit to see what response it had at this particular setting.

This setting is 5dB above unity which is to say that if you hit the bypass switch, volume will drop by 5dB.  





At these two settings the EQ and function of the ZROCK2 remain largely the same.  In the 200K load, there is a slight presence peak of about 2dB and then a small roll off right at 20KHz.  In the 24K load it is the same but without the small treble roll off.

Both have a 3dB increase at 20Hz which is only 1dB at 40Hz, so basically not really heard.  

So my regular setting is by and large a flat response at 5dB above unity.  



Above is the setting for the ZROCK2 video demo using the classic rock band Boston.  You can see we are now at close to 9dB above unity with a bass shelf of 6dB above that.  Also in this setting there is a treble rise into the lower 24K impedance shown by the blue line.

Here is a link the video demo of the ZROCK2 playing Boston with the above settings, both on and off.

https://decware.wistia.com/medias/pifb52r0ad

I can't answer your specific question but you can put the ZR2 before the preamp.